Tighter emission standards and significant innovation in catalyst formulations and engine controls has led to dramatic improvements in the low emission performance and robustness of gasoline and diesel engine systems. However, many technical challenges remain to make the conventionally fueled internal combustion engine a nearly zero emission system having the efficiency necessary to make the vehicle commercially viable.
The automotive industry has made very significant progress in reducing automotive emissions in both the mandated test procedures and the “real world”. This has resulted in some added cost and complexity of engine management systems, yet those costs are offset by other advantages of computer controls: increased power density, fuel efficiency, drivability, reliability and real-time diagnostics.
Future initiatives to require zero emission vehicles appear to be taking us into a new regulatory paradigm where asymptotically smaller environmental benefits come at a very large incremental cost. Yet, even an “ultra low emission” certified vehicle may emit high emissions in limited extreme ambient and operating conditions or with failed or degraded components, and especially during cold start.
One approach to addressing the issue of emissions is the employment of fuel cells, particularly solid oxide fuel cells (“SOFC”), in an automobile either as a primary or secondary source of power. A fuel cell is an energy conversion device that generates electricity and heat by electrochemically combining a gaseous fuel, such as hydrogen, carbon monoxide, or a hydrocarbon, and an oxidant, such as air or oxygen, across an ion-conducting electrolyte. The fuel cell converts chemical energy into electrical energy. SOFCs are constructed entirely of solid-state materials, utilizing an ion conductive oxide ceramic as the electrolyte. An electrochemical cell in a SOFC may comprise an anode and a cathode with an electrolyte disposed there between. The oxidant passes over the oxygen electrode (cathode) while the fuel passes over the fuel electrode (anode), generating electricity, water, and heat. The use of the SOFC, and fuel cells in general, reduce emissions through their much greater efficiency, and so require less fuel for the same amount of power produced, as compared to conventional hydrocarbon fueled engines. Additionally, a fuel cell may be employed to supplement a conventional engine; in this way the engine may be optimized for primary traction power, while the fuel cell may provide other power needs for the vehicle, i.e. air-conditioner, communication and entertainment devices. Additionally, the fuel reformer—fuel cell system may be operated while the engine is off, permitting electrically powered devices to operate, and so further reducing emissions by providing power using a more fuel efficient fuel cell to meet the vehicle operator's needs.
To facilitate the production of electricity by the SOFC, a direct supply of simple fuel, e.g., hydrogen, carbon monoxide, and/or methane is preferred. However, concentrated supplies of these fuels are generally expensive and difficult to supply. Therefore, the fuel utilized may be obtained by processing a more complex fuel source. The actual fuel utilized in the system is chosen based upon the application, expense, availability, and environmental issues relating to the fuel. Possible fuels include hydrocarbon fuels, including, but not limited to, liquid fuels, such as gasoline, diesel fuel, ethanol, methanol, kerosene, and others; gaseous fuels, such as natural gas, propane, butane, and others; “alternative” fuels, such as biofuels, dimethyl ether, and others; synthetic fuels, such as synthetic fuels produced from methane, methanol, coal gasification or natural gas conversion to liquids, and combinations comprising at least one of the foregoing methods, and the like; as well as combinations comprising at least one of the foregoing fuels. The preferred fuel is based upon the types of equipment employed, with lighter fuels, i.e., those that may be more readily vaporized and/or conventional fuels, which are readily available to consumers being generally preferred.
Processing or reforming of hydrocarbon fuels, such as gasoline, may be completed to provide an immediate fuel source for rapid start up of the fuel cell as well as protecting the fuel cell by breaking down long chain hydrocarbons and by removing impurities. Fuel reforming may include mixing fuel with air, water and/or steam in a reforming zone before entering the reformer system, and converting a hydrocarbon (such as gasoline) or an oxygenated fuel (such as methanol) into hydrogen (H2) and byproducts (e.g. carbon monoxide (CO), carbon dioxide (CO2) methane (CH4), inert materials (e.g., nitrogen (N2), carbon dioxide (CO2), and water (H2O)). Approaches to reforming include steam reforming, partial oxidation, dry reforming, and combinations comprising at least one of the foregoing. Both steam reforming and dry reforming are endothermic processes, while partial oxidation is an exothermic process.
Accordingly, a SOFC may be used in conjunction with a fuel reformer to convert a hydrocarbon-based fuel to hydrogen and carbon monoxide (the reformate) usable by a fuel cell. Preferably, the reformer has a rapid start, a dynamic response time, and excellent fuel conversion efficiency. It is also preferred for the reformer to have a minimal size and reduced weight, as compared to other power sources. However, reformers operate at temperatures that may be greater than about 600° C., and may exceed about 1000° C. or more. At lower temperatures, for example during start-up, deposition of carbonaceous matter (or soot) upon the catalyst may adversely affect the reformer's efficiency, reduce reformer life, and/or damage fuel cell components. Accordingly, it is beneficial to reduce the time required by a reformer and/or fuel cell system to reach an operational temperature.